Image processing apparatus, control method thereof, and storage medium

ABSTRACT

The occurrence of image quality adverse effects, such as interference fringes and color bleeding, is suppressed for an object including a spot color adjustment-target color. In a case where a spot color adjustment function is made use of, the same color separation table is caused to be applied uniformly, instead of switching color separation tables to be applied to an adjustment-target color depending on whether the object attribute is text/line/graphics, or image.

FIELD OF THE DISCLOSURE

The aspect of the embodiments relates to a color adjustment technique atthe time of performing printing processing based on image data.

DESCRIPTION OF THE RELATED ART

For the use to obtain a printed material including a company logo or aproduct logo, such as POP (Point of purchase advertising) printing, highcolor reproducibility is required for a specific color, such as aso-called corporate color. In this regard, accompanying the improvementof performance of a recent household or office image forming apparatus,the POP printing or the like is performed in each shop or the like.Then, in the recent household or office image forming apparatus, it ismade possible for a user without particular expertise to easily matchcolors. Japanese Patent Laid-Open No. 2017-22648 has disclosed atechnique to perform color correction of image data so that a specificcolor in an image to be printed on a printing medium becomes close to adestination color without using a dedicated color measurement device.The color adjustment that takes the specific color in a printing-targetimage as a target is called “spot color adjustment” in thisspecification.

Printing-target image data that is transmitted from a PC or the like isusually represented in the sRGB color space, which is the standard colorspace. Because of this, at the time of performing printing in an imageforming apparatus, the sRGB color space is converted into the CMYK colorspace that the image forming apparatus can reproduce. Then, at the timeof this conversion, a conversion table (color separation LUT) inaccordance with the attribute of an object is used. Here, in many cases,the object having a color that is the target of the above-described spotcolor adjustment is an object whose area that is represented in the samecolor is comparatively wide. In a case where a color separation LUT isselected in accordance with the attribute for the object such as thiswhich includes the spot color adjustment-target color, it happenssometimes that interference fringes and color bleeding are elicited inthe object.

SUMMARY OF THE DISCLOSURE

The apparatus includes: a first conversion table that convertsdevice-independent RGB values into device-dependent RGB values; secondand third conversion tables that convert device-dependent RGB valuesinto CMYK values; a conversion unit configured to convert, for pixelsconstituting an input image, device-dependent RGB values converted bythe first conversion table: into CMYK values by using the secondconversion table for a constituent pixel whose object attribute is text;and into CMYK values by using the third conversion table for aconstituent pixel whose object attribute is image; and a reception unitconfigured to receive instructions to change a certain color included inthe input image into another color, and in a case where the receptionunit receives the instructions, the conversion unit performs conversioninto CMYK values indicating said another color by using a fixedconversion table irrespective of object attribute thereof for aconstituent pixel having the certain color.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a hardware configuration of an MFP;

FIG. 2 is a function block diagram showing an internal configuration ofan image processing unit;

FIG. 3 is a function block diagram showing an internal configuration ofa color correction table generation unit according to a firstembodiment;

FIG. 4 is a flowchart showing a flow of color correction tablegeneration processing;

FIG. 5A to FIG. 5C are each a diagram showing a UI screen 500 forsetting a destination color;

FIG. 6 is a flowchart showing a flow of color conversion processing in aspot color adjustment mode;

FIG. 7 is a function block diagram showing an internal configuration ofa color correction table generation unit according to a secondembodiment;

FIG. 8 is a flowchart showing details of second table creationprocessing according to the second embodiment;

FIG. 9 is a function block diagram showing an internal configuration ofa color correction table generation unit according to a thirdembodiment; and

FIG. 10 is a flowchart showing details of second table creationprocessing according to the third embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, with reference to the attached drawings, the presentdisclosure is explained in detail in accordance with embodiments.Configurations shown in the following embodiments are merely exemplaryand the present disclosure is not limited to the configurations shownschematically.

In each embodiment, explanation is given by taking a so-called MFP(Multi Function Printer) as an example of an image forming apparatusthat forms an image in the electrophotographic method on a printingmedium in accordance with printing-target image data. However, theapplication range of the technique of the present disclosure is notlimited to the MFP and it is possible to apply the technique widely to ageneral image output apparatus that outputs an image in accordance withinput image data. That is, it is also possible to apply the technique toan image display device, such as a monitor and a projector, in additionto other kinds of image forming apparatus, such as a copy machine, alaser printer, and an ink jet printer.

Before explaining each embodiment of the technique of the presentdisclosure, the background of the technique is explained in detail. In acase of a general household or office image forming apparatus, there arerestrictions on the number of color materials that can be mounted andthe color material amount (toner amount or ink amount) that can beformed on a printing medium. Because of this, it is difficult to preparea color conversion table with which it is possible to performappropriate color reproduction at all times for all printing-targetimages. Consequently, a method is adopted conventionally in which aplurality of color conversion tables limiting image quality items towhich priority is given is prepared in advance and an optimum colorconversion table is selected in accordance with a printing-target imagethat is input. For example, in a color separation table that is used ina case where the device-dependent RGB color space is converted into theCMYK color space corresponding to the color materials, replacement ofmixed black (process black) including cyan, magenta, and yellow withsingle black is also performed at the same time. A plurality of colorseparation tables whose replacement ratio at that time is varied isprepared in advance and a color separation table suitable to eachattribute is selected and applied in accordance with the attribute of anobject included in the printing-target image. In this case, for thecolor separation table whose replacement ratio is high, the number ofcolor materials to be used is suppressed, and therefore, image qualityadverse effects, such as interference fringes and color bleeding, becomemore likely to occur in a large area. On the other hand, such a colorseparation table has a feature that the influence of a registrationshift at an edge portion is small, and therefore, is advantageous informing a high-definition image whose sharpness is high. On thecontrary, in a case of a color separation table whose replacement ratiois low, there is a tendency that the number of color materials to beused increases, and therefore, the amount of consumed color materialsincreases and the sharpness is reduced, but there is a feature that sucha color separation table has a wide color reproduction range and isadvantageous in drawing a vivid image. By taking into consideration thefeatures such as those, a method of selecting a color separation tablesuitable to each attribute in accordance with the attribute of an objectincluded in the printing-target image is adopted. Specifically, thecolor separation table whose replacement ratio is high (graycompensation type) is applied to the object area of the text/lineattribute and graphics attribute, for which priority is given tosharpness. On the contrary, the color separation table whose replacementratio is low (non-gray compensation type) is applied to the object areaof the image attribute, for which priority is given to colorreproducibility. In the following, the “text/line attribute” is simplydescribed as “text attribute”.

Here, a relationship with the above-described spot color adjustmentbecomes problematic. The spot color adjustment functions in the colormatching processing to convert the values (sRGB values) in thedevice-independent RGB color space of the input image into values(devRGB values) in the device-dependent RGB color space. For example, itis assumed that the sRGB values of (R, B, B)=(255, 0, 0) representingred are converted into devRGB values of (R, G, B)=(240, 0, 0) in thenormal color matching processing. In a case of the spot color adjustmentthat takes red as an adjustment-target color, the red is converted intospecified red designated by a user, whose devRGB values are, forexample, (R, G, B)=(240, 10, 3).

Here, the issue is that a plurality of (here, two) color conversiontables exists as described above, which performs conversion from thedevice-dependent RGB color space into the CMYK color space. For example,in a case where R=G=B=100, color conversion is performed as follows inthe above-described two kinds of color separation table.

gray compensation type: K=155, C=M=Y=0

non-gray compensation type: K=105, C=M=Y=50

The above-described fact means that even though an attempt is made toreproduce a desired color by performing the spot color adjustment in theconversion from the sRGB color space into the devRGB color space, themicroscopic tint changes depending on which type of color separationtable is applied in the subsequent color separation processing. In moredetail, in a case where the gray compensation type whose replacementratio from the mixed black into the single black is high is applied, theK component becomes more likely to be recognized visually as colorbleeding or interference fringes.

First Embodiment

<Hardware Configuration of Image Forming Apparatus>

FIG. 1 is a block diagram showing the hardware configuration of an MFPas an image forming apparatus according to the present embodiment. AnMFP 100 comprises a CPU 101, a ROM 102, a RAM 103, a large-capacitystorage device 104, a UI unit 105, an image processing unit 106, anengine interface (I/F) 107, a network I/F 108, and a scanner I/F 109.Each of these units is connected to one another via a system bus 110.Further, the MFP 100 comprises a printer engine 111 and a scanner unit112. The printer engine 111 and the scanner unit 112 are connected tothe system bus 110 via the engine I/F 107 and the scanner I/F 109,respectively. The image processing unit 106 may be configured as animage processing apparatus (image processing controller) independent ofthe MFP 100.

The CPU 101 controls the operation of the entire MFP 100. The CPU 101performs various kinds of processing, to be described later, by readingprograms stored in the ROM 102 onto the RAM 103 and executing theprograms. The ROM 102 is a read-only memory and in the ROM 102, thesystem activation program or the program for performing the control ofthe printer engine, and character data or character code information andthe like are stored. The RAM 103 is a volatile random access memory andused as a work area of the CPU 101 and a temporary storage area ofvarious kinds of data. For example, the RAM 103 is used as a storagearea for storing font data registered additionally by download or imagefiles received from the external apparatus, and the like. Thelarge-capacity storage device 104 is, for example, an HDD and an SSD andin which various kinds of data are spooled and which is used for storingvarious tables, information files, image data and the like and used as awork area.

The UI (user interface) unit 105 includes, for example, a liquid crystaldisplay (LCD) comprising a touch panel and displays the setting state ofthe MFP 100, the situation of the processing being performed, the errorstate and the like. For example, the UI unit 105 is also used to displaythe adjustment-target color candidate and the destination colorcandidate at the time of performing color adjustment processing.Further, the UI unit 105 receives various user instructions, such as theinput of values in various settings of the MFP 100, selection of variousbuttons and the like. For example, the setting relating to the spotcolor adjustment processing, execution instructions and the like areperformed via the UI unit 105. It may also be possible for the UI unit105 to separately comprise an input device, such as a hard key.

The engine I/F 107 functions as an interface for controlling the printerengine 111 in response to instructions from the CPU 101. Via the engineI/F 107, transmission and reception of engine control commands and thelike are performed between the CPU 101 and the printer engine 111. Thenetwork I/F 108 functions as an interface for connecting the MFP 100 tothe network 113. The network 108 may be, for example, a LAN or thetelephone line network (PSTN). The printer engine 111 forms amulti-colored image on a printing medium, such as paper, by using colormaterials (here, toner) of a plurality of colors (here, four colors ofCMYK) based on the print image data received from the side of the systembus 110. The scanner I/F 109 functions as an interface for controllingthe scanner unit 112 in response to instructions from the CPU 101 at thetime of performing reading of a document by the scanner unit 112. Viathe scanner I/F 109, transmission and reception of scanner unit controlcommands and the like are performed between the CPU 101 and the scannerunit 112. The scanner unit 112 generates read image data by reading adocument optically under the control of the CPU 101 and transmits theread image data (scanned image data) to the RAM 103 or thelarge-capacity storage device 104 via the scanner I/F 109.

In the present embodiment, a case is supposed where, for example,printing is performed in the printer engine 11 by using input image datafor POP printing, but the tint in the printing results is different fromthat of the sample printed material. A user having recognized that thetint in the printing results is different from that of the sampleprinted material operates the UI unit 105 to select the spot coloradjustment operation mode and performs the setting of the color forwhich the user desires to perform adjustment (adjustment-target color)and the color aimed at by the user (destination color). Then, it isassumed that based on the user operation such as this, a dedicated colorconversion table (in the following, described as “color correctiontable”) having the conversion characteristic that causes theadjustment-target color included in the input image data to bereproduced by the destination color is generated and the printingprocessing involving the above-described spot color adjustment isperformed. That is, the generation of the color correction table forspot color adjustment is performed in a case where explicit instructionsare input from a user.

<Configuration of Image Processing Unit>

FIG. 2 is a function block diagram showing the internal configuration ofthe image processing unit 106. The image processing unit 106 comprisesan image input unit 201, a control command generation unit 202, a colorcorrection table generation unit 203, a color conversion unit 204, a RIPunit 205, a halftone processing unit 206, and an image output unit 207.Each of these function units is implemented by the CPU 101 readingprograms stored in the ROM 102 onto the RAM 103 and executing theprograms. Alternatively, part or all of these units may be implementedby hardware, such as an ASIC or an electronic circuit.

The image input unit 201 receives the input of printing-target imagedata. The image data that is input is input from, for example, the hostPC 115 via the network 113 and the network I/F 108. Alternatively, theimage data stored in the large-capacity storage device 104 may be input.The image data that is input is two-dimensional data including the planeof each color of red (R), green (G), and blue (B), in which the colorsignal corresponding to the sRGB color space independent of the printerengine 111 is represented by eight bits (256 tones). Here, sRGB refersto the standard of the RGB color space determined by IEC (InternationalElectrotechnical Commission). The image data that is input to the imageinput unit 201 is sent to the control command generation unit 202.

The control command generation unit 202 acquires image data in the CMYKcolor space that the printer engine 111 can handle by controlling thecolor correction table generation unit 203 and the color conversion unit204. Then, the control command generation unit 202 generates a commandfor controlling the RIP unit 205 (in the following, described as “RIPcontrol command”), which includes the obtained image data in the CMYKcolor space. The generated RIP control command is sent to the RIP unit205.

The color correction table generation unit 203 generates theabove-described color correction table for replacing theadjustment-target color of the input image data into the destinationcolor under the control of the control command generation unit 202. Thecolor correction table is a table that converts the color in thedevice-independent sRGB color space into the color in the CMYK colorspace dependent on the color reproducibility of the printer engine 111.By this color correction table, in the color conversion unit 204, theinput color signal (sRGB signal) corresponding to the adjustment-targetcolor designated by a user is converted into the output color signal(CMYK signal) that implements the destination color desired by a user ona printing medium. Details of color correction table generationprocessing will be described later. The generated color correction tableis stored in the RAM 103 or the large-capacity storage device 104 andthe color conversion unit 204 reads and uses the color correction tableat the time of the spot color adjustment mode setting.

The color conversion unit 204 performs color conversion processing forthe printing-target input image data by using the color matching LUT andthe color separation LUT, which are prepared in advance. “LUT” is anabbreviation of lookup table. Here, the outline thereof is explained.First, in the normal printing mode, sRGB values are converted intodevRGB values in accordance with the color matching LUT. Then, theconverted devRGB values are converted into CMYK values by selectivelyapplying the color separation LUT of gray compensation type or non-graycompensation type in accordance with the object attribute. On the otherhand, in the spot color adjustment mode, by using the color correctiontable generated by the color correction table generation unit 203, thesRGB values corresponding to the adjustment-target color included in theinput image data are converted directly into CMYK values capable ofimplementing the destination color. That is, in a case of the presentembodiment, for the object of the spot color adjustment-target color,the control is performed so that the color conversion of non-graycompensation type is applied at all times irrespective of the attributeof the object. However, in a case also where the spot color adjustmentmode is selected, for the object of a color other than theadjustment-target color, the same processing as that in the normalprinting mode is performed. That is, after the conversion into thedevRGB values by the color matching LUT, the color separation LUT ofgray compensation type or non-gray compensation type is applied inaccordance with the object attribute. Details of the color conversionprocessing in the spot color adjustment mode will be described later.The image data is finally obtained in which each pixel is represented byCMYK values by taking the sRGB values of the input image data as a startpoint, and therefore, a configuration may be accepted in which theabove-described color matching processing and color separationprocessing are performed by using a function formula in place of thetable. In either configuration, the color separation of non-graycompensation type is to be applied fixedly irrespective of the attributefor the object having the spot color adjustment-target color.

The RIP (Raster Image Processor) unit 205 generates image data in theraster format represented in the CMYK color space (in the following,described as “CMYK raster image data”) by using the RIP unit controlcommand generated by the control command generation unit 202.

The halftone processing unit 206 generates print image data (halftoneimage data) represented by halftone dots, which the printer engine 111can process, by performing halftone processing for the CMYK raster imagedata generated by the RIP unit 205. In many cases, it is normallypossible for the printer engine 111 to output image data with a numberof tones smaller than that of the input image data, such as two tones,four tones, and 16 tones. Because of this, the halftone processing isperformed so that it is possible to represent a stable halftone even ina case where the image data is printed and output with a smaller numberof tones as described above. The method of halftone processing includesthe density pattern method, the systematic dither method, the errordiffusion method and the like and it is possible to apply these publiclyknown methods.

In a case of receiving print image data from the halftone processingunit 206, the image output unit 207 transmits the print image data tothe printer engine 111 via the engine I/F 107. The CPU 100 givesinstructions to form an image based on the print image data to theprinter engine 111. The printer engine 111 forms and outputs a colorimage in accordance with the input print image data onto a printingmedium by performing each process, such as exposure, development,transfer, and fixing.

<Color Correction Table Generation Processing>

Following the above, color correction table generation processing isexplained in detail, which is a feature of the present embodiment. FIG.3 is a function block diagram showing the internal configuration of thecolor correction table generation unit 203 according to the presentembodiment and FIG. 4 is a flowchart showing a flow of the operationthereof. The color correction table generation unit 203 comprises anadjustment-target color setting unit 301, a destination color settingunit 302, a first table creation unit 303, a second table creation unit304, and a table combination unit 305. Each of these function units isimplemented by the CPU 101 reading a predetermined program stored in theROM 102 onto the RAM 103 and executing the program. In the following,explanation is given along the flowchart in FIG. 4.

At S401, the adjustment-target color setting unit 301 sets a targetcolor of spot color adjustment. Specifically, first, in a case whereprinting-target image data is input from the control command generationunit 202, the image data is displayed on the UI unit 105. Then, thecolor signal values (sRGB values) corresponding to a position designatedby a user with a mouse or the like for the displayed image are acquiredand the acquired sRGB values are set as an adjustment-target colortgt_sRGB. Information on the set adjustment-target color is sent to thefirst table creation unit 303.

At S402 that follows, the destination color setting unit 302 sets adestination color for the adjustment-target color that is set at S401.FIG. 5A to FIG. 5C are each a diagram showing an example of a UI screen500 for destination color setting. With reference to FIG. 5A to FIG. 5C,a destination color setting method is explained. First, as in FIG. 5A, amessage 502 prompting a user to read a sample document 501 including adestination color is displayed on the UI screen 500. In a case where auser sets the sample document 501 on the scanner unit 112 and givesinstructions to start reading, the sample document 501 is read and imagedata is generated. This image data is 8-bit image data represented inthe RGB color space (scanRGB) dependent on the scanner unit 112. Theimage obtained by reading the sample document 501 is displayed in apreview area 503 on the UI screen 500 as a sample image 504 as in FIG.5B and at the same time, the image data thereof is stored in the RAM103. Then, in a case where a user designates a desired position 505 witha mouse or the like in the sample image 504 within the preview area 503as in FIG. 5C, the color corresponding to the position 505 is displayedin a destination color display field 506. Then, in a case where a userpresses down a Determine button 507. the color signal values (scanRGBvalues) representing the color being displayed in the destination colordisplay field 506 are set as a destination color dest_scanRGB. Thedestination color setting method is not limited to the example describedabove. For example, it may also be possible to set a destination colorby displaying colors included in the sample image 504 in a list and by auser selecting a destination color form the list. Further, it may alsobe possible to directly set RGB values specifying a destination colorwithout reading the sample document 501. Information on the setdestination color is sent to the first table creation unit 303.

At S403, the first table creation unit 303 creates a first table thatassociates the adjustment-target color tgt_sRGB that is set at S401 withvalues (devRGB values) in the RGB color space dependent on the printerengine 111, which correspond to the destination color dest_scanRGB thatis set at S402. At this time, a conversion LUT 311 as shown in Table 1below is used, which is prepared in advance and which associates scanRGBvalues and devRGB values with each other. This conversion LUT 311 isacquired by reading it from, for example, the large-capacity storagedevice 104. However, it may also be possible to acquire the table datafrom an external apparatus, such as the host PC 115, via the network113.

TABLE 1 Input (scanRGB) Output (devRGB) R G B R G B 0 0 0 0 0 0 0 0 16 05 15 0 0 32 0 10 30 0 0 48 0 10 45 0 0 64 0 10 60 0 0 80 0 10 80 0 0 960 12 100 . . . . . . . . . . . . . . . . . .

In a case where the destination color dest_scanRGB is converted intodevRGB values by using the conversion LUT 311 as shown in Table 1described above, the first table as shown in Table 2 below is created,which associates the adjustment-target color tgt_sRGB and the devRGBvalues corresponding to the destination color dest_scanRGB with eachother.

TABLE 2 Input (sRGB) Output (devRGB) R G B R G B 255 0 0 255 10 20

The first table acquired as described above is sent to the tablecombination unit 305. Further, information on the devRGB valuescorresponding to the destination color dest_scanRGB is sent to thesecond table creation unit 304.

Following the above, at S404, the second table creation unit 304 createsa second table that associates the devRGB values corresponding to thedestination color dest_scanRGB received from the first table creationunit 303 with the CMYK values corresponding to the color material usedin the printing processing. At this time, a color separation LUT 312 ofnon-gray compensation type that associates the devRGB values and theCMYK values with each other as shown in Table 3 below is used. That is,as described previously, among a plurality of color separation LUTsprepared in advance, the color separation LUT of non-gray compensationtype that is excellent in color reproducibility and unlikely to causeimage quality adverse effects, such as interference fringes and colorbleeding.

TABLE 3 Input (devRGB) Output (CMYK) R G B C M Y K 0 0  0 30.0 30.0 5.0100.0 0 0 16 50.0 50.0 5.0 85.0 0 0 32 60.0 60.0 3.0 80.0 . . . . . . .. . . . . . . . . . . . . . 255  255  255  0 0 0 0

Here, the reason the color separation LUT of non-gray compensation typeis used is explain anew in detail. As described previously, as the colorseparation LUT, a plurality of types is prepared whose ratio in whichthe process black is replaced with single black (replacement ratio intoK color material) is different. Then, in a case where the objectattribute is text or graphics, priority is given to sharpness and byapplying a color separation LUT whose replacement ratio into K colormaterial is high, a printed image whose edge of the object is sharp isimplemented. On the other hand, in a case where the object attribute isimage, priority is given to color reproducibility and by applying acolor separation LUT whose replacement ratio into K color material islow, a printed image excellent in balance of colors and rich in colorreproduction is implemented. Further, a color separation LUT is designedso that the color material amounts of colors whose distances betweengrid points are short change as little as possible in order to suppressa pseudo contour that may occur in a case where an object having agradation representation is drawn. Because of this, for the colorseparation LUT whose replacement ratio into K color material, in thegrid point group located in the vicinity of the area that is representedby the K color material, the number and the amount of color materials tobe used are limited. Because of this, image quality adverse effects,such as interference fringes and color bleeding, are likely to occur.Here, the interference fringes mean a fringe pattern that occurs in acase where the image (color plane) of each color material interfereswith another on a printing medium. In an image forming apparatus thatrepresents a so-called area gradation, each color plane that is formedon a printing medium has a different periodicity for each color. In acase where each color plane is viewed individually, the periodicity isat a level that human eyes cannot perceive, but in a case where two ormore color planes overlap, they interfere with one another and theperiodicity becomes a level that can be perceived, and therefore,visually recognized as a fringe pattern. The more the overlapping colorplanes become in number, the less conspicuous the interference fringessuch as these become because the fringes occur randomly. Because ofthis, as in the above-described grid point group in the color separationLUT whose replacement ratio into K color material is high, in the areathat is represented by two colors including the K color material, theinterference fringes become more likely to be conspicuous. Further,there is also an issue of color bleeding. The color bleeding (or alsorepresented as “color becomes turbid”) refers to the state whereportions whose brightness difference is large are interspersed withinthe area of uniform color. By the portions likely to be visuallyrecognized as colors whose brightness difference is large beinginterspersed within the area supposed to be uniform, a user recognizethis as image quality adverse effects. In a case where the replacementratio into K color material in the color separation LUT is increased, inthe vicinity of the area that is represented by the K color material, inorder to suppress the sudden change in the color material amount, alarge amount of K color material and a small amount of another colormaterial are combined frequently. Because of this, in the area in whichblack color is predominant, a small amount of cyan, magenta, or yellowwhose brightness is lower than that of black color is mixed, andtherefore, the color bleeding becomes more likely to occur. Then, thelarger the brightness difference, the higher the visual recognizabilityof the color bleeding becomes, and therefore, with the combination ofblack and yellow that maximizes the brightness difference, the colorbleeding becomes most obvious. The image quality adverse effects, suchas the interference fringes and the color bleeding described above, areunevenness in a case a uniform color is represented and have such afeature that the image quality adverse effects become likely to beobvious in particular in the low-frequency area in which a color isrepresented uniformly in a wide area and are hardly recognized visuallyin the high-frequency area, such as a thin line. The above-describedissue is not inherent to the area that is represented by the K colormaterial. For example, in a case where importance is given to sharpness,a single-color representation is used frequency in order to suppress thecolor material amount, and therefore, the same issue occurs also in thearea that is represented by another color material color. With the abovein mind, in the present embodiment, even in a case where the objectattribute is text or graphics, color reproduction is taken as thehighest priority item and the color separation LUT of non-graycompensation type whose replacement ratio into K color material is lowis used at the time of creation of the second table. The colorseparation LUT 312 shown in FIG. 3 is acquired by reading it from, forexample, the large-capacity storage device 104, but it may also bepossible to acquire the color separation LUT 312 from an externalapparatus, such as the host PC 115, via the network 113.

As above, at S404, by converting the devRGB values corresponding to thedestination color dest_scanRGB into the CMYK values using the colorseparation LUT of non-gray compensation type, the second table as shownin Table 4 below is created.

TABLE 4 Input (devRGB) Output (CMYK) R G B C M Y K 255 10 20 0.0 95.095.0 10.0

The second table created as described above is sent to the tablecombination unit 305.

Next, at S405, the table combination unit 305 obtains a color correctiontable as shown in Table 5 below by combining the first table receivedfrom the first table creation unit 303 and the second table receivedfrom the second table creation unit 304.

TABLE 5 Input (sRGB) Output (CMYK) R G B C M Y K 255 0 0 0.0 95.0 95.010.0

In this manner, a color correction table is obtained, which associatesthe sRGB values of the adjustment-target color and the CMYK valuescorresponding to the destination color with each other and whichincludes the conversion characteristic of the color separation LUT ofnon-gray compensation type. The generated color correction table isstored in the RAM 103 or the large-capacity storage device 104 and atthe same time, the control command generation unit 202 is notified ofthat the generation of the color correction table is completed.

The above is the contents of the color correction table generationprocessing according to the present embodiment. In the presentembodiment, the devRGB values corresponding to the destination colordest_scanRGB are converted into the CMYK values by using the colorseparation LT whose replacement ratio into K color material is lower ofthe two kinds of color separation LUT of different type, but the presentembodiment is not limited to this. What is required is to be capable ofavoiding application of the color separation LUT whose replacement ratiointo K color material is high and which has a high risk of theoccurrence of image quality adverse effects at the time of conversioninto the CMYK values corresponding to the destination color. Forexample, in a case where there are three kinds of color separation LUTdifferent in the replacement ratio into K color material, it may also bepossible to create the second table by adopting the color conversiontable whose replacement ratio into K color material is intermediate.

<Color Conversion Processing>

Next, color conversion processing that is performed in the colorconversion unit 204 in the spot color adjustment mode is explained withreference to the flowchart in FIG. 6. The series of processing shown inthe flow in FIG. 6 is implemented by the CPU 101 reading a predeterminedprogram stored in the ROM 102 onto the RAM 103 and executing theprogram.

At S601, data of various tables used in the color conversion unit 204 isread from the RAM 103 or the large-capacity storage device 104.Specifically, in addition to the color correction table generated by thecolor correction table generation unit 203, the color matching LUTprepared in advance and the two kinds (gray compensation type andnon-gray compensation type) of color separation LUT different in thereplacement ratio into K color material are read.

At S602 that follows, the pixel of interest in the printing-target inputimage is determined. At this time, the pixel is determined as the pixelof interest in order from, for example, the pixel in the top-left cornerof the input image.

At S603 that follows, whether or not the sRGB values, which are thepixel values of the pixel of interest, are equal to the input values inthe color correction table that is read at S601. In a case where thedetermination results indicate that both are equal, the processingadvances to S604 and in a case where both are not equal, the processingadvances to S605.

At S604, the pixel values of the pixel of interest are converted inaccordance with the color correction table. That is, the sRGB valuesspecified as the input values in the color correction table areconverted into the CMYK values specified as the output values thereof.In a case where the conversion processing is completed, the processingadvances to S609.

At S605, the sRGB values, which are the pixel values of the pixel ofinterest, are converted in accordance with the color matching LUT. Thatis, input values corresponding to the sRGB values of the pixel ofinterest are specified among the plurality of input values specified inthe color matching LUT and converted into the devRGB values as theoutput values associated with the input values. In a case where the sRGBvalues that match with the sRGB values of the pixel of interest are notspecified as input values in the color matching LUT, it is sufficient todetermine the devRGB values corresponding to the sRGB values of thepixel of interest by publicly known interpolation processing. In a casewhere the conversion processing into the devRGB values is completed, theprocessing advances to S606.

At S606, whether or not the object attribute of the pixel of interest istext or graphics is determined. In a case where the object attribute istext or graphics, the processing advances to S607 and in a case wherethe object attribute is not text or graphics (that is, the objectattribute is image), the processing advances to S608.

At S607, the pixel values of the pixel of interest converted at S605 areconverted in accordance with the color separation LUT of graycompensation type. That is, input values corresponding to the devRGBvalues of the pixel of interest are specified among the plurality ofinput values specified in the color separation LUT whose replacementratio into K color material is higher and converted into the CMYK valuesas the output values associated with the input values. Similarly, atS608, the pixel values of the pixel of interest converted at S605 areconverted in accordance with the color separation LUT of non-graycompensation type. That is, input values corresponding to the devRGBvalues of the pixel of interest are specified among the plurality ofinput values specified in the color separation LUT whose replacementratio into K color material is lower and converted into the CMYK valuesas the output values associated with the input values. In a case wherethe devRGB values that match with the devRGB values of the pixel ofinterest are not specified as the input values in the color separationLUT, it is sufficient to determine by publicly known interpolationprocessing and this is the same as at S605. In a case where theconversion processing into the CMYK values is completed, the processingadvances to S609.

At S609, whether or not the above-described processing is completed forall the pixels included in the printing-target input image isdetermined. In a case where there is an unprocessed pixel, theprocessing returns to S602, and the next pixel of interest is determinedand the processing is continued. On the other hand, in a case where allthe pixels have been processed, this processing is terminated.

The above is the contents of the color conversion processing in the spotcolor adjustment mode according to the present embodiment.

Modification Example 1

In the spot color adjustment of the present embodiment, the colorconversion is performed once by using the conversion table that directlyassociates the sRGB values of the adjustment-target color and the CMYKvalues of the destination color with each other, but the spot coloradjustment is not limited to this. For example, the configuration may beone in which color conversion is performed in two stages by using thefirst table before combination and the color separation LUT 312 ofnon-gray compensation type. Further, it may also be possible tointerpose a process of temporarily converting the sRGB values of theadjustment-target color into the Lab values, and then, converting theLab values into the CMYK values. Here, “Lab” is a simplifiedrepresentation of “L*a*b*” that is the three-dimensional visual uniformcolor space independent of the printer engine 111, which takes intoconsideration the human visual characteristic, determined by CIE(International Commission on Illumination). What is required is thatconversion based on the color separation LUT whose replacement ratiointo K color material is low be performed without depending on theobject attribute in the process of performing the conversion of theadjustment-target color into the CMYK values representing thedestination color.

Modification Example 2

It is possible to widely apply the method of the present embodiment tothe suppression of image quality adverse effects resulting from thereplacement ratio being different at the time of replacing a mixed color(process color) represented by a combination of a plurality of colormaterials with a single color represented by one color material. Forexample, it is assumed that the printer engine 111 comprises a bluecolor material, whose color is an intermediate color of the processcolor, and a plurality of color separation LUTs whose replacement ratioat the time of replacing the mixed color of cyan and magenta with theblue color material, whose color is a single color, is prepared inadvance so as to be used in accordance with the object attribute. In thecase such as this also, it is possible to apply the present embodimentand it is possible to obtain the same effect by causing the colorseparation LUT whose replacement ratio into the blue color material islower to be applied at the time of performing the spot color adjustment.Further, the color material of an intermediate color may be any colormaterial capable of representing an intermediate hue of the processcolor, such as orange and violet, in addition to blue.

As above, according to the present embodiment, as regards the objecthaving a spot color adjustment-target color, a fixed color separationLUT is applied irrespective of the attribute thereof and conversion intothe CMYK values is performed. Because of this, it is possible tosuppress the occurrence of image quality adverse effects, such asinterference fringes and color turbidity, in the object of the spotcolor adjustment-target color.

Second Embodiment

In the first embodiment, at the time of generating the color correctiontable for spot color adjustment, the CMYK values are determined by usingthe color separation LUT of non-gray compensation type at all times.Next, an aspect is explained as a second embodiment in which a colorseparation LUT that is used at the time of generating a color correctiontable is determined based on the feature amount of the destination colorthat is set along with the spot color adjustment-target color.Explanation of the contents common to those of the first embodiment,such as the hardware configuration of the MFP, is omitted and in thefollowing, color correction table generation processing, which is adifferent point, is explained.

FIG. 7 is a function block diagram showing the internal configuration ofa color correction table generation unit 203′ according to the presentembodiment. The color correction table generation unit 203′ newlycomprises a feature amount derivation unit 701, in addition to theadjustment-target color setting unit 301, the destination color settingunit 302, the first table creation unit 303, a second table creationunit 304′, and the table combination unit 305. The feature amountderivation unit 701 derives the saturation of the destination color thatis set by a user as the feature amount and outputs obtained saturationinformation to the second table creation unit 304′. Then, the secondtable creation unit 304′ of the present embodiment determines the typeof the color separation LUT that is used at the time of creating thesecond table based on the saturation information received from thefeature amount derivation unit 701. In more detail, in a case where thedestination color designated by a user is an achromatic color that isunlikely to cause image quality adverse effects to occur even though thereplacement ratio into K color material is high, the second table iscreated by using the color separation LUT of gray compensation type.

The general flow of the color correction table generation processing isas shown in the flowchart in FIG. 4 described previously also in thepresent embodiment and there is almost no difference from that of thefirst embodiment. The difference from the first embodiment is the secondtable creation method (that is, the contents at S404 in the flow in FIG.4). FIG. 8 is a flowchart showing the flow of the operation in thesecond table creation unit 304′ of the present embodiment. In thefollowing, along the flowchart in FIG. 8, the second table creationmethod in the present embodiment is explained.

At S801, the information on the destination color (scanRGB values) thatis set at S402 in the flow in FIG. 4 is acquired. The acquiredinformation on the destination color is input to the feature amountderivation unit 701. At S802 that follows, the feature amount derivationunit 701 converts the scanRGB values acquired at S801 into Lab values byusing the conversion LUT prepared in advance and derives saturation S asthe feature amount from the obtained Lab values. The color conversionLUT that is used here is a conversion LUT similar to the conversion LUTshown in Table 1 described previously, which associates the RGB colorspace dependent on the scanner unit 112 and the device-independent Labcolor space. Here, the Euclid distance between the a value and the bvalue in the Lab values corresponding to the destination color obtainedby the conversion is found as the saturation S. Information on thesaturation S thus obtained as the feature amount is sent to the secondtable creation unit 304′.

Then, at S803, the second table creation unit 304′ determines whetherthe destination color obtained at S801 is an achromatic color based onthe saturation S derived at S802. Specifically, the second tablecreation unit 304′ performs threshold value determination processing todetermine whether the saturation S derived at S802 is smaller than athreshold value Th_s provided in advance. As the threshold value Th_s atthis time, it is sufficient to adopt the upper limit value of thesaturation corresponding to the grid point that is reproduced by the Kcolor material in the color separation LUT of gray compensation typewhose replacement ratio into K color material is high. In a case wherethe determination results indicate that the saturation S is smaller thanthe threshold value Th_s, it is determined that the set destinationcolor is a color included in the achromatic area and the processingadvances to S804. On the other hand, in a case where the saturation S islarger than or equal to the threshold value Th_s, it is determined thatthe set destination color is not a color included in the achromatic area(that is, chromatic color) and the processing advances to S805.

At S804 and S805, the second table creation unit 304′ selects a colorseparation LUT to be used in accordance with the results of thethreshold value processing based on the saturation information andcreates the second table. At S804 in a case where it is determined thatthe destination color is an achromatic color, based on the colorseparation LUT of gray compensation type whose replacement ratio into Kcolor material is high (that is, achromatic color reproducibility ishigh), the second table that associates the devRGB values correspondingto the destination color dest_scanRGB with the CMYK values is created.On the other hand, at S805 in a case where it is determined that thedestination color is not an achromatic color, as at S404 of the firstembodiment, based on the color conversion table of non-gray compensationtype whose replacement ratio into K color material is low, the secondtable that associates the devRGB values corresponding to the destinationcolor dest_scanRGB with the CMYK values is created.

The above is the contents of the second table creation processingaccording to the present embodiment. In the present embodiment, in acase where the destination color is an achromatic color, by using thecolor separation LUT whose replacement ratio into K color material ishigh, the CMYK values specified as the output values in the second tableare determined. Due to this, it is made possible to represent anachromatic color object vividly while suppressing the occurrence ofimage quality adverse effects.

Third Embodiment

In the second embodiment, the aspect is explained in which the type ofthe color separation LUT that is used at the time of creation of thesecond table is determined in accordance with whether the destinationcolor in the spot color adjustment is an achromatic color. Next, anaspect is explained as a third embodiment in which the type of a colorseparation LUT that is used at the time of creation of the second tableis determined in accordance with visual sensitivity of a pixel areahaving a spot color adjustment-target color. As in the secondembodiment, explanation of the contents common to those of the firstembodiment, such as the hardware configuration of the MFP, is omittedand in the following, color correction table generation processing,which is a different point, is explained.

FIG. 9 is a function block diagram showing the internal configuration ofa color correction table generation unit 203″ according to the presentembodiment. The color correction table generation unit 203″ newlycomprises a feature amount derivation unit 701′, in addition to theadjustment-target color setting unit 301, the destination color settingunit 302, the first table creation unit 303, a second table creationunit 304″, and the table combination unit 305. The feature amountderivation unit 701′ derives the spatial frequency of a pixel areahaving an adjustment-target color within a printing-target image as afeature amount and outputs obtained frequency information to the secondtable creation unit 304″. Then, the second table creation unit 304″ ofthe present embodiment determines the type of a color separation LUTthat is used at the time of creation of the second table based on thefrequency information received from the feature amount derivation unit701′. In more detail, in a case where the pixel area of theadjustment-target color is a high-frequency area in which the finerepresentation is predominant, image quality adverse effects areunlikely to occur even though the replacement ratio into K colormaterial is high, and therefore, the second table is created by usingthe color separation LUT of gray compensation type.

The general flow of the color correction table generation processing isas shown in the flowchart in FIG. 4 described previously also in thepresent embodiment, and there is almost no difference from that of thefirst and second embodiments. The difference from the first and secondembodiments is the second table generation method (that is, the contentsat S404 in the flow in FIG. 4). FIG. 10 is a flowchart showing the flowof the operation in the second table creation unit 304″ of the presentembodiment. In the following, along the flowchart in FIG. 10, the secondtable creation method in the present embodiment is explained.

At S1001, the data of the input image for which the setting of theadjustment-target color has been performed at S401 described previouslyis acquired. The acquired input image data is input to the featureamount derivation unit 701′. At S1002 that follows, coordinateinformation indicating the pixel position in the input image of thepixel having the adjustment-target color that has been set at S401described previously is acquired. The acquired input image data is inputto the feature amount derivation unit 701′.

At S1003 that follows, the feature amount derivation unit 701′ derivesthe spatial frequency of the pixel area having the adjustment-targetcolor among the pixels within the input image as the feature amount.Specifically, first, each pixel constituting the input image isprocessed sequentially and whether or not the pixel has the color thatmatches with the adjustment-target color is determined. By attaching “1”to the pixel determined to have the color that matches with theadjustment-target color as a result of the determination and “0” to thepixel determined to have a color that does not match therewith, a 1-bitplane image (binary image) having the same size as that of the inputimage is generated. Next, by performing the publicly known Fouriertransform processing for the generated binary image, the amplitude ofeach frequency that the adjustment-target color included in the inputimage has is found. Then, the frequency having the maximum amplitude istaken as a representative frequency f and this is taken as the featureamount. Then, information on the representative frequency f thusobtained as the feature amount is sent to the second table creation unit304″.

Then, at 51004, the second table creation unit 304″ determines whetheror not the pixel area of the adjustment-target color is a high-frequencyarea. Specifically, the second table creation unit 304″ performsthreshold value determination processing to determine whether therepresentative frequency f derived at S1003 is smaller than a thresholdvalue Th_f provided in advance. As regards the threshold value Th_f, itis sufficient for a user to check the chart image with his/her nakedeye, which is obtained by performing conversion into CMYK values for achart image including areas of a variety of frequencies by using thecolor separation LUT whose replacement ratio into K color material ishigh and then printing and outputting the chart image, and adopt thefrequency that is unlikely to cause interference fringes and colorbleeding to occur as the threshold value Th_f. In a case where thedetermination results indicate that the representative frequency f ishigher than the threshold value Th_f, it is determined that the pixelarea having the set adjustment-target color is an area in which priorityis given to sharpness over color reproducibility and the processingadvances to S1005. On the other hand, in a case where the representativefrequency f is lower than or equal to the threshold value Th_f, it isdetermined that the pixel area having the set adjustment-target color isan area in which priority is given to color reproducibility oversharpness and the processing advances to S1006.

At S1005 and S1006, the second table creation unit 304″ creates thesecond table by selecting a color separation LUT to be used inaccordance with the results of the threshold value processing based onthe frequency information. At S1005 in a case where the pixel area ofthe adjustment-target color is determined to be a high-frequency area,the second table is created based on the color separation LUT of graycompensation type whose replacement ratio into K color material is high.On the other hand, at S1006 in a case where the pixel area of theadjustment-target color is determined not to be a high-frequency area,as at S404 of the first embodiment, the second table is created based onthe color conversion table of non-gray compensation type whosereplacement ratio into K color material is low.

The above is the contents of the second table creation processingaccording to the present embodiment. What is required is to be capableof specifying the spatial frequency of the pixel area having theadjustment-target color as the feature amount, and therefore, forexample, it may also be possible to find the connectivity and the areaof the pixel including the adjustment-target color and estimate thespatial frequency of the pixel area having the adjustment-target colorfrom the obtained connectivity and the area.

As above, in the present embodiment, in a case where the pixel areahaving the adjustment-target color is a high-frequency area, the CMYKvalues that are specified as the output values in the second table aredetermined by using the color separation LUT whose replacement ratiointo K color material is high. Due to this, it is made possible torepresent an object whose representation is fine more vividly whilesuppressing the occurrence of image quality adverse effects.

Other Embodiments

Embodiment(s) of the present disclosure can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

It is made possible to suppress the occurrence of image quality adverseeffects, such as interference fringes and color bleeding, for an objectincluding a spot color adjustment-target color.

This application claims the benefit of Japanese Patent Application No.2020-083782, filed May 12, 2020 which is hereby incorporated byreference wherein in its entirety.

What is claimed is:
 1. An apparatus comprising: a first conversion tablethat converts device-independent RGB values into device-dependent RGBvalues; second and third conversion tables that convert device-dependentRGB values into CMYK values; a conversion unit configured to convert,for pixels constituting an input image, device-dependent RGB valuesconverted by the first conversion table: into CMYK values by using thesecond conversion table for a constituent pixel whose object attributeis text; and into CMYK values by using the third conversion table for aconstituent pixel whose object attribute is image; and a reception unitconfigured to receive instructions to change a certain color included inthe input image into another color, wherein in a case where thereception unit receives the instructions, the conversion unit performsconversion into CMYK values indicating said another color by using afixed conversion table irrespective of object attribute thereof for aconstituent pixel having the certain color.
 2. The apparatus accordingto claim 1, wherein the fixed conversion table is the third conversiontable.
 3. The apparatus according to claim 1, wherein the conversionunit uses, as the fixed conversion table, a conversion table including aconversion characteristic of the third conversion table, which takesdevice-independent RGB values corresponding to the certain color asinput values and CMYK values corresponding to said another color asoutput values.
 4. The apparatus according to claim 1, wherein theconversion unit uses, as the fixed conversion table, a fourth conversiontable that converts device-independent RGB values corresponding to thecertain color into device-dependent RGB values and the third conversiontable.
 5. The apparatus according to claim 1, wherein the conversionunit uses, as the fixed conversion table, a conversion table including aconversion characteristic of one of the second and third conversiontables that take device-independent RGB values corresponding to thecertain color as input values and CMYK values corresponding to saidanother color as output values, which is selected in accordance withsaturation of said another color.
 6. The apparatus according to claim 5,wherein in a case where the saturation of said another color indicatesan achromatic color, the third conversion table is selected and in acase where said another color indicates a chromatic color, the secondconversion table is selected.
 7. The apparatus according to claim 1,wherein the conversion unit uses, as the fixed conversion table, aconversion table including a conversion characteristic of one of thesecond and third conversion tables that take device-independent RGBvalues corresponding to the certain color as input values and CMYKvalues corresponding to said another color as output values, which isselected in accordance with visual sensitivity of a pixel area havingthe certain color.
 8. The apparatus according to claim 7, wherein in acase where a spatial frequency of the pixel area having the certaincolor is higher than a predetermined threshold value, the thirdconversion table is selected and in a case where the spatial frequencyof the pixel area having the certain color is lower than or equal to thepredetermined threshold value, the second conversion table is selected.9. The apparatus according to claim 1, wherein the second conversiontable and the third conversion table are different in a replacementratio in a case where a mixed color is replaced with a single color andthe replacement ratio in the third conversion table is lower than thereplacement ratio in the second conversion table.
 10. The apparatusaccording to claim 9, wherein the mixed color is mixed black and thesingle color is single black.
 11. The apparatus according to claim 9,wherein the mixed color is a process color represented by combining atleast two colors of cyan, magenta, and yellow and the single color is anintermediate color of the process color.
 12. A method of an apparatuscomprising: a first conversion table that converts device-independentRGB values into device-dependent RGB values; and second and thirdconversion tables that convert device-dependent RGB values into CMYKvalues, the method comprising: converting, for pixels constituting aninput image, device-dependent RGB values converted by the firstconversion table: into CMYK values by using the second conversion tablefor a constituent pixel whose object attribute is text; and into CMYKvalues by using the third conversion table for a constituent pixel whoseobject attribute is image; and performing, in a case of receivinginstructions to change a certain color included in the input image intoanother color, conversion into CMYK values indicating said another colorby using a fixed conversion table irrespective of object attributethereof for a constituent pixel having the certain color.
 13. The methodaccording to claim 12, wherein the converting uses, as the fixedconversion table, a conversion table including a conversioncharacteristic of the third conversion table, which takesdevice-independent RGB values corresponding to the certain color asinput values and CMYK values corresponding to said another color asoutput values.
 14. The method according to claim 12, wherein theconverting uses, as the fixed conversion table, a fourth conversiontable that converts device-independent RGB values corresponding to thecertain color into device-dependent RGB values and the third conversiontable.
 15. The method according to claim 12, wherein the convertinguses, as the fixed conversion table, a conversion table including aconversion characteristic of one of the second and third conversiontables that take device-independent RGB values corresponding to thecertain color as input values and CMYK values corresponding to saidanother color as output values, which is selected in accordance withsaturation of said another color.
 16. A non-transitory computer readablestorage medium storing a program for causing a computer to perform amethod of an apparatus comprising: a first conversion table thatconverts device-independent RGB values into device-dependent RGB values;and second and third conversion tables that convert device-dependent RGBvalues into CMYK values, the control method comprising the steps of:converting, for pixels constituting an input image, device-dependent RGBvalues converted by the first conversion table: into CMYK values byusing the second conversion table for a constituent pixel whose objectattribute is text; and into CMYK values by using the third conversiontable for a constituent pixel whose object attribute is image; andperforming, in a case of receiving instructions to change a certaincolor included in the input image into another color, conversion intoCMYK values indicating said another color by using a fixed conversiontable irrespective of object attribute thereof for a constituent pixelhaving the certain color.
 17. The non-transitory computer readablestorage medium according to claim 16, wherein the converting uses, asthe fixed conversion table, a conversion table including a conversioncharacteristic of the third conversion table, which takesdevice-independent RGB values corresponding to the certain color asinput values and CMYK values corresponding to said another color asoutput values.
 18. The non-transitory computer readable storage mediumaccording to claim 16, wherein the converting uses, as the fixedconversion table, a fourth conversion table that convertsdevice-independent RGB values corresponding to the certain color intodevice-dependent RGB values and the third conversion table.
 19. Thenon-transitory computer readable storage medium according to claim 16,wherein the converting uses, as the fixed conversion table, a conversiontable including a conversion characteristic of one of the second andthird conversion tables that take device-independent RGB valuescorresponding to the certain color as input values and CMYK valuescorresponding to said another color as output values, which is selectedin accordance with saturation of said another color.